Update documentation.rst

docs/sphinx/contributing/documentation.rst

@@ -36,7 +36,7 @@ manager, you can install them using the Python package manager ``pip``:
 
 .. code-block:: bash
 
-   pip install --user "sphinx=5.3" sphinx-book-theme breathe sphinxcontrib-bibtex sphinx-copybutton
+   pip install --user "sphinx==5.3" sphinx-book-theme breathe sphinxcontrib-bibtex sphinx-copybutton
 
 You may need to set the following CMake variables to make sure CMake can
 find the required dependencies.